1. Chimpanzee and human cultures have been permanently altered from normal to preneoplastic using hybrid Ad12-SV40 tumor virus. In one instance normal chimpanzee skin has been successfully transformed to an established malignant chimpanzee line (sarcoma). 2. Mouse leukemia retrovirus recombinants between the "experimental" Rauscher and the "endogenous" xenotropic strains behave as the fully virulent leukemogenic Rauscher parent in immunodeficient hosts, but are much less pathogenic or non-pathogenic in immunocompetent siblings. 3. Human and animal xenografts on athymic nude mice have been used to study tumor progression and malignant transformation. 4. Psoralen-UV photochemical inactivation of tumor viruses before infecting cells has produced less cytopathic viral transformation.